The aim of this study is to obtain information on the genetic control of immunoglobulin synthesis and on the synthesis of cell surface antigens through studies on cultured cells and on man-mouse somatic cell hybrids. Most assays for Ig synthesis involve the precipitation of newly synthesized immunoglobulin by the addition of antisera or Straphylococcus aureus and antisera. For proper interpretation of results of these studies it is essential to determine which molecules synthesized by cells in culture may bind to Ig aggregates and to Staph. aureus organims, "non-specifically". Studies in progress are aimed at further characterizing such material and determining its relationship to immunoglobulins , structural cellular proteins and transplantation antigens. In characterizing these molecules monoclonal antibodies will be used for isolation of materials under study, and precipitated material will be examined by peptide mapping.